Danfoss Cascade HFC Application Manual Download Page 3

Cascade HC/HFC - CO2 system 

Application guide  RA8AB102  ©  Danfoss  10/2010 

3

Temperatures and pressures 
in cascade systems (contin-
ued)

On systems with low temperatures of the dis-
charge CO2 gas (low superheat), the superheat 
of the expansion valve can be the dimensioning 
factor for the heat exchanger. 
If a CO2 system has high superheat, then desu-
per-heaters need to be used in order to reduce 
the load on the high temperature side.

Optimal intermediate pressure in CO2 cascade 
systems depends on a number of parameters 
(high temperature refrigerant, load pattern etc.). 
Generally 2 cases need to be considered:

Operating sequence of cas-
cade systems

1) Systems with load at the medium temperature. 
In this case intermediate pressure should be as 
high as possible in order to reduce the load at 
the high temperature stage. The limitations are 
therefore required temperature on the intermedi-
ate level and pressure rating of the system.

2) Systems without load at medium temperature.
In this case the intermediate temperature should 
be in the range of -10 - 0°C (due to the high pres-
sure of the CO2 LT) where lower limit is defined 

by efficiency and higher by system pressure 
rating

In Cascade Systems, it is essential that at least 
one compressor in the high temperature side is 
running before the first compressor in the low 
temperature side can start. Otherwise, the com-
pressor in the low temperature side may be cut 
out due to high pressure. 

The same sequence is also valid for filling up the 
system. First of all, the high temperature circuit 
needs to be filled with refrigerant and started up. 
When this is done, the CO2 can be filled into the 

low temperature system.

The high temperature expansion valve (ETS) to 
the cascade heat exchanger should begin simul-
taneously with the high temperature compres-
sors. After this, the valve controls the superheat 
of the high temperature gas. LT compressors are 
then started up by the CO2 pressure increase on 

the suction line.

Danfoss pack controllers such as AK-PC 740 and 
AK-PC 780 are specially designed with built in 
control functions to coordinate these operations.

Injection into cascade heat 
exchanger

Injecting liquid into a plate heat exchanger is 
not a trivial matter. The heat exchanger is often 
compact and therefore the time constant is very 
low. AKV valves are not recommended for this 
application. 

It is recommended to use motor valves or other 
valves that give constant flow. Desuper-heating 
of CO2 gas entering the cascade heat exchanger 

can also be recommended for three reasons. 

One reason is that the gas is often 60°C and 
therefore the heat can be rejected to the ambient 
or used for heat recovery without problems. The 
second reason is to reduce thermal stress in the 
heat exchanger. The third reason is that the CO2 

gas gives very high heat fluxes which therefore 
create unstable conditions on the evaporation 
side. Therefore it is recommended to reduce the 
superheat on the CO2 side.

Distribution on the CO2 side is also a critical 

issue. This is why the heat exchanger has to be 
designed for direct expansion to make sure the 
mixture of gas and liquid is evenly distributed to 
the heat exchanger.

When the heat exchanger is designed for reason-
able pressure drop at part load, the oil transport 
and distribution should work under most condi-
tions.

Summary of Contents for Cascade HFC

Page 1: ...R AKS 32R GD DGS AKS 11 AKS 2050 AKS 11 AK PC 740 780 GD DGS AKS 11 AKS 32R AKD 102 AKS 11 DCR Pump SGN AKS 2050 AKS 11 EKC 313 EVR ETS DCR AKD 102 AKD 102 AK CC 450 AKS 11 AKVH AK CC 550 A AKS 11 AK...

Page 2: ...scharge pressure setting 30 bar 5 C The higher the efficiency of the cascade heat exchanger the lower the difference between the condensation temperature of CO2 and the evaporating temperature of the...

Page 3: ...d with refrigerant and started up When this is done the CO2 can be filled into the low temperature system The high temperature expansion valve ETS to the cascade heat exchanger should begin simul tane...

Page 4: ...2 R AKS 32R GD DGS AKS 11 AKS 2050 AKS 11 AK PC 740 780 GD DGS AKS 11 AKS 32R AKD 102 AKS 11 DCR Pump SGN AKS 2050 AKS 11 EKC 313 EVR ETS DCR AKD 102 AKD 102 AK CC 450 AKS 11 AKVH AK CC 550 A AKS 11 A...

Page 5: ...n the AK PC 740 780 make it possible to use the Pc pressure in the LT CO2 side as control sensor for the HT suction pressure This to ensure fast and stable control of the condens ing pressure of LT CO...

Page 6: ...medium temperature display cases or rooms is controlled by opening closing a solenoid valve motor valve in the CO2 supply to the evaporator The actual temperature control can take place in two ways as...

Page 7: ...n evaporator perform ance Stepper motor driver for ETS or CCM expansion valve 0 10 V output for ICMTS valves Electrically operated expansion valves for precise liquid injection in evaporators Fully ba...

Page 8: ...Danfoss reserves the right to alter its products without notice This also applies to products already on order provided that such alternations can be made without subsequential changes being necessar...

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